The objective of this research is to arrive at a detailed understanding of the catalytic mechanism of the enzyme DNA ligase by synthesizing several new NAD ion analogues which have had their adenylylphosphorous atoms deactivated to nucleophilic attack by the substitution of an adjacent oxygen atom by an element or group which is less electronegative. The substituted elements or groups will be nitrogen, esterified and non-esterified sulfur, and a methylene group. These analogues will be used with the enzyme to help determine whether the covalent enzyme-AMP is a true intermediate in the reaction or a reversibly formed side product. In addition the enzyme RNA ligase has been purified and it will be characterized with regard to substrate specificity and mechanism. These two enzymes which join the preformed pieces of nucleic acid will be compared and contrasted with one another. The effects of the triphosphate derivatives of 3'-deoxyadenosine and 3'-deoxyuridine upon DNA synthesis in E. coli cells rendered permeable by treatment with toluene will be examined.